Report the current or last self-upgrade.

Phases: idle (there has been none), pulling, recreating (this API is about to be replaced, so the next answer comes from the new one), done, failed. After a successful recreate the record is the one this API collected from the helper container on boot, including its exit code and the tail of its output.

Returns: Dictionary with run_id, phase, detail, previous_digest, new_digest, exit_code and log_tail.